This invention relates generally to improvements in dental instruments particularly for use in placing retraction cord into the gingival crevice surrounding a selected tooth. More specifically, this invention relates to an improved retractor instrument designed for facilitated placement of retraction cord while preventing or minimizing trauma to adjacent gingival tissue.
Dental restorations such as crowns and bridges are well-known in the art, wherein the exposed crown portion of one or more natural teeth is replaced by a prosthesis formed typically from a gold alloy which may carry an outer shell of porcelain or the like. In accordance with common restorative procedures, the exposed crown portion of the natural tooth is reduced and shaped by drilling to remove damaged portions and to define a suitable prepared base for supporting a prosthetic crown. In this regard, the natural tooth is normally trimmed to a so-called margin disposed slightly below the gum line defined by the surrounding gingival tissue, wherein this margin constitutes a line of separation between cut and uncut portions of the tooth. A moldable and curable impression material such as a selected vinyl-based elastomer is then placed over and about the prepared tooth and allowed to cure, resulting in a resilient mold or impression which is intended to replicate the prepared tooth and surrounding structures within the patient's mouth. This impression is then used according to known casting techniques to produce a custom prosthesis adapted to seat securely onto the prepared tooth and to mate comfortably with adjacent teeth. The prosthesis is installed by cementing onto the prepared tooth in an essentially permanent manner.
The marginal fit between the prosthetic crown and the natural tooth is critical in order to obtain prolonged service life without decay of the prepared tooth in the marginal area. In this regard, to achieve the desired accuracy in marginal detail, it is extremely important for the impression material to flow to and at least slightly beyond the margin of the prepared tooth to insure full reproduction of the marginal area. Unfortunately, since the tooth margin is recessed below the surrounding gum line, the gingival tissue normally blocks flow of the impression material to the desired location. Moreover, trimming of the tooth to the recessed margin is usually accompanied by at least some gingival bleeding around the tooth periphery, wherein the resultant body fluids can also block the desired flow of the impression material.
In the past, these problems have been addressed by the use of retraction cord for temporarily separating the gingival tissue from the margin of the prepared tooth. More specifically, subsequent to tooth preparation and before making the impression, a flexible retraction cord or string is normally placed by packing into the so-called gingival crevice surrounding the prepared tooth. This retraction cord is thus placed slightly below the gum line and functions as a spacer to separate the gingival tissue from the tooth in the marginal area. In many cases, the retraction cord is preimpregnated with a chemical agent that assists in retracting the gingival tissue while reducing the tendency of the tissue to contract immediately when the cord is removed. The retraction cord is then removed immediately prior to making the impression, thereby permitting the impression material to contact and reproduce the marginal area of the tooth.
While the use of retraction cord beneficially enhances the quality of the impression, placement of the retraction cord can be a difficult and time consuming procedure. That is, the gingival tissue normally fits closely about the tooth to define a gingival crest at the gum line. From the gingival crest, the tissue descends alongside the tooth to define a shallow gingival crevice or sulcus, typically having a depth on the order of 1.8 to 2.0 millimeters, the base of which includes periodontal attachment tissue secured to the tooth. The retraction cord must be packed into the gingival crevice to separate the tissue lining the crevice from the tooth, but without disturbing or traumatizing the closely underlying periodontal attachment tissue. In the past, this packing procedure has been performed with a dental instrument having a narrow blunted tip with a squared or convex profile by pressing the instrument tip against the cord to push the cord incrementally into the gingival crevice. More recently, an instrument having a rotatable blunted tip has been proposed. See U.S. Pat. No. 4,396,375. In either case, the blunted tip is intended to avoid trauma to the gingival tissue. However, the blunted tip does not securely engage the retraction cord and thus tends to slip off the cord to contact and damage gingival tissue, particularly such as the periodontal attachment tissue at the base of the gingival crevice. When this attachment tissue is damaged in this manner, the tissue tends to heal with at least some gum line recession resulting in potential exposure of the marginal area between the prosthetic crown and the natural tooth.
There exists, therefore, a significant need for improvements in dental instruments of the type used for placing retraction cord into the gingival crevice surrounding a selected tooth, substantially without damaging the adjacent gingival tissue. The present invention fulfills this need and provides further related advantages.